Buried pipelines are often coated and/or subjected to cathodic protection (CP) techniques in an effort to mitigate corrosion. However, even the best of today's corrosion avoidance techniques do not prevent corrosion at some point. Coating defects develop and buried pipelines corrode. In the pipeline industry, such defects in the pipeline coating are known as “holidays”.
Buried pipelines are not easily accessible for holiday detection. Thus, special techniques have been developed for detecting coating holidays without having to expose the pipe surface.
One approach to detecting coating defects is electrochemical impedance spectroscopy (EIS). EIS methods measure the impedance of a system over a range of frequencies, and therefore the frequency response of the system, including the energy storage (capacitive) and dissipation (resistive) properties, is revealed. Often, data obtained by EIS is expressed graphically in a Bode plot or a Nyquist plot. For pipeline applications, EIS has been studied previously, but many challenges have arisen in analyzing EIS data gathered in field studies.
“Local” EIS (LEIS) is conducted by applying a sinusoidal potential signal (excitation signal) between the pipeline and a buried counter electrode, as with EIS. The response is measured from two reference electrodes (rather than one as in EIS) placed in the soil above a section of the pipeline to detect defects in that section.